The present invention claims priority from Japanese Patent Application No. 11-356020 filed Dec. 15, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to radio communications, and particularly to radio communications in which the conditions of radio wave propagation vary over time. More particularly, the present invention relates to a diversity technique for use in interoffice radio communications, interbuilding communications, or satellite communications, in which temporal variations in radio wave propagation characteristics due to fading are not substantial.
2. Description of Related Art
In line-of-sight point-to-point communications using micro waves or millimeter waves, space diversity communication systems are widely used as effective means for avoiding temporal variations in radio wave propagation characteristics due to fading. Also in mobile communications such as portable telephones, diversity communication systems are widely used in order to avoid temporal variations in radio wave propagation characteristics due to multipath fading. In these conventional diversity communication systems, the transmission side device defines two radio wave propagation paths for carrying one transmission signal, and the reception side device uses two independent receivers for receiving the transmitted signals via the two radio wave propagation paths, respectively, thereby obtaining two reception outputs. Then, the two reception outputs are diversity-combined with each other. Diversity maximum ratio combining is a known technique used for combining the reception outputs together.
FIG. 6 is a block diagram illustrating a conventional device for use in such systems. Single transmission data is provided to a modulator 301 of the transmission side device. The modulated output signal diverges into two signals of the same information, which are then input to two independent transmitters 302 and 303, respectively. Local oscillators 304 and 305 provide carrier frequencies to the two transmitters 302 and 303, respectively, for frequency conversion of the two signals. The signals are then transmitted to two propagation paths 306 and 307.
The signals from the two propagation paths 306 and 307 are received by receivers 308 and 309, respectively, in the reception side device. Local oscillators 310 and 311 provide carrier frequencies to the receivers 308 and 309, respectively, for frequency de-conversion of the received signals. The signals are then subjected to a gain adjustment process by two automatic gain controllers (AGCs) 312 and 313, respectively. The outputs from the automatic gain controllers are subjected to a multiplication operation by complex multipliers 316 and 317 with a weighting coefficient which is calculated by correlation control circuits 314 and 315, respectively. The two output signals are added together by an addition circuit 318 into a single signal. Each of the two correlation control circuits 314 and 315 generates the weighting coefficient based on the output from the addition circuit 318 so that the output signal from the addition circuit 318 will have an amplitude that is in proportion to the square of each of the signals received via the two propagation paths and be in phase with the received signals. Thus, a diversity maximum ratio combining operation is performed, and reception data corresponding to the transmission data is obtained by a demodulator 319.
For example, JP S59-105727A describes a reception side structure for diversity communications based on maximum ratio combining.
However, with the conventional diversity communication systems as described above, while two independent radio wave propagation paths are occupied, the total signal transmission capacity is equivalent to that achieved by a system which uses only one radio wave propagation path. Thus, the radio wave resources are not efficiently used except when the characteristics of one of the radio wave propagation paths has degraded to a point where the path is substantially unusable.
The hardware resources of the transmitter/receiver are also not used efficiently. Specifically, while each of the transmission side device and the reception side device is provided with two independent sets of hardware equipment, only a single signal can be transmitted between the transmission side and the reception side.
The present invention has been made in view of such circumstances in the prior art, and it is an object of the present invention to provide a diversity communication system with which the signal transmission capacity can be increased if the quality of the propagation paths is kept at a certain level. It is another object of the present invention to provide a diversity communication device with which two different signals can be transmitted while providing diversity maximum ratio combining for each of the signals, as long as each of the two independent propagation paths maintains a certain level of quality. It is a further object of the present invention to improve the efficiency of use of radio wave resources and hardware resources.
In order to achieve these objects, according to the present invention, the transmission side device combines together two transmission signals independent of each other so as to produce two combined signals which are vector-wise in a mirror image relationship with respect to each other, and transmits the two combined signals over two propagation paths, respectively. The reception side device receives the combined signals (or xe2x80x9creception signalsxe2x80x9d) and restores the two independent transmission signals therefrom, based on the mirror image relationship between the reception signals, by eliminating one of the transmission signals from one of the reception signals and eliminating the other one of the transmission signals from the other one of the reception signals.
In other words, the present invention is based on a principle that if two transmission signals (i.e., a first transmission signal and a second transmission signal) are combined together to produce two reception signals to be transmitted over two propagation paths, respectively, so that the produced reception signals are in a mirror image relationship (i.e., so that the second transmission signal, for example, in one reception signal has a vector which is oppositely oriented with respect to that of the second transmission signal in the other reception signal with the vector of the first transmission signal of one reception signal being aligned with that of the first transmission signal of the other reception signal), then, the second transmission signal can be eliminated on the reception side by using the two reception signals. Based on such a principle, the transmission side device combines two transmission signals together so that the obtained reception signals are vector-wise in a mirror image relationship with respect to each other, and transmits the obtained reception signals. The reception side device restores each transmission signal by eliminating the other transmission signal which has been superimposed on the transmission signal to be restored. In this way, the present invention realizes a transmission capacity that is up to twice as much as that achieved by conventional diversity communication systems.
Specifically, according to the present invention, the transmission side device combines two independent transmission signals (S1, S2) together to produce two combined signals, i.e., a sum signal (S1+S2) and a difference signal (S1xe2x88x92S2), and transmits the produced signals over two propagation paths, respectively. After the sum signal and the difference signal are received by the reception side device, the sum signal and the difference signal, as received, are subjected to an AGC control operation and a complex-level multiplication operation with a weighting coefficient based on correlation control, and then separated into two, restored, independent signals.
More specifically, the present invention provides a diversity transmission/reception device, comprising a transmission side device and a reception side device, the transmission side device comprising: addition means for receiving two transmission signals (S1 and S2) independent of each other to produce an output signal (S1+S2); subtraction means for receiving the two transmission signals (S1 and S2) independent of each other to produce an output signal (S1xe2x88x92S2); first transmission means for diversity-transmitting the output signal (S1+S2) from the addition means to a first propagation path as a first reception signal; and second transmission means for diversity-transmitting the output signal (S1xe2x88x92S2) from the subtraction means to a second propagation path as a second reception signal, and the reception side device comprising: first and second reception means for receiving the first and second reception signals which have passed through the first and second propagation paths, respectively; first and second automatic gain controlling means for receiving, as their inputs, the outputs from the first and second reception means, respectively; first and second multiplication means for subjecting the output signals from the first and second automatic gain controlling means to complex multiplication with weighting coefficients (W1 and W2) so as to produce output signals (x1 and x2), respectively; addition means and subtraction means receiving, as their inputs, the output signals (x1 and x2) from the first and second multiplication means so as to produce outputs (y1=x1+x2) and (y2=x1xe2x88x92x2), respectively; first correlation control means for performing a correlation operation between a first input and a second input, the first input being the output (y1=x1+x2) from the addition means and the second input being the output from the first automatic gain controlling means, so as to provide the weighting coefficient (W1) to the first multiplication means; and second correlation control means for performing a correlation operation between a first input and a second input, the first input being the output (y2=x1xe2x88x92x2) from the subtraction means and the second input being the output from the second automatic gain controlling means, so as to provide the weighting coefficient (W2) to the second multiplication means.
With such a structure, as long as each of the two propagation paths maintains reasonable transmission characteristics, it is possible to obtain diversity effects deriving from the use of two propagation paths, and to increase the transmission capacity by a factor of up to two, thereby improving the efficiency of use of radio wave frequency resources and hardware resources.
In the present invention, the two transmission signals (S1 and S2) independent of each other can be signals which are obtained by modulating two carrier waves of the same frequency whose phases are shifted from each other by about xcfx80/2. Thus, when carrier waves are modulated respectively by two modulators with two input transmission data signals, it is preferred that the two modulators have the same carrier frequency and phases that are shifted from each other by about xcfx80/2. The phrase xe2x80x9cabout xcfx80/2xe2x80x9d as used herein means that the phase difference does not need to be exactly xcfx80/2, but some deviation in phase is acceptable as long as the diversity operation of the present invention can be performed.
Such a structure enables the reception side device to perform a diversity maximum ratio combining process. Moreover, with such a structure, two independent propagation paths can be used for two phase-multiplexed signals. As a result of the phase multiplexing, an extra room available for the transmission of more information is created in the transmission band which would otherwise be fully occupied. Such an extra capacity can be used for realizing diversity effects.
The first and second correlation control means may comprise means for calculating the weighting coefficients (W1 and W2), respectively, so that each of the output (y1) from the addition means and the output (y2) from the subtraction means of the reception side device has an amplitude in proportion to the square of a respective one of the first and second reception signals and is in phase with the reception signal.
Most suitably, the gains of the first and second automatic gain controlling means are in proportion to 1/|xcfx811| and 1/|xcfx812|, respectively (where xcfx811 and xcfx812, each being a complex number, denote attenuation coefficients of the first and second propagation paths, respectively), while the weighting coefficients (W1 and W2) are W1=k1xcfx811*/|xcfx811| and W2=k2xcfx812*/|xcfx812|, respectively (where k1 and k2 denote proportional constants, and xcfx811* and xcfx812* denote complex conjugates of xcfx811 and xcfx812, respectively).
Alternatively, the first and second reception signals which have been demodulated by demodulation means may be re-modulated to produce re-modulated signals, so as to provide the re-modulated signals, instead of the outputs from the addition means and the subtraction means, to the first and second correlation control means, respectively, as their reference signals. In such a case, receiver noise, or the like, is not contained in the reference signals which are provided for the correlation calculation, whereby it is possible to perform an accurate correlation operation.
Alternatively, preamble signals from preamble signal generation means may be provided, instead of the outputs from the addition means and the subtraction means, to the first and second correlation control means, respectively, as their reference signals. Alternatively, preamble signals extracted form the respective outputs from the first and second automatic gain controlling means may be provided to the first and second correlation control means, respectively, for the correlation operation.
Also in such cases, it is possible to perform an accurate correlation operation because the operation would not then be influenced by receiver noise, or the like.
It is preferred to provide means for monitoring characteristics of the propagation paths, so that when a quality of one of the propagation paths has degraded thereby reducing the reception signal level to a point where the reception signal can no longer be reproduced, the monitoring means can instruct the transmission side device to stop the transmission signal combining operation, thereby stopping the transmission signal combining operation on the transmission side. Thus, even if the reception signal level significantly decreases for one propagation path, for example, it is possible to avoid situations where neither of the transmission signals can be restored.